Conventionally, in vehicles, particularly in small two-wheeled vehicles and the like, ACG (AC generator) starter motors acting as starter motors at start of engines and acting also as power generators after the start of the engines are often used. Three-phase DC brushless motors are used for these ACG starter motors (Patent Document 1). For detection of a rotor position at the start of the engine and at the time of power generation of this three-phase DC brushless motor, hall sensors have been provided for respective phase windings of the three-phase winding, and thereby a rotor position has been detected using the hall sensors.
On the other hand, a technique of detecting a rotor position without using the hall sensors has been developed (Patent Document 2). In a control device described in Patent Document 2, when the brushless motor is stopped, a DC voltage with a plurality of patterns is applied to two windings of the three-phase windings for a short period of time, and rise characteristics of current which varies in accordance with the rotor position are detected, thereby detecting the stop position of the rotor.